With respect to microcomputer video signals and transmission thereof, one early protocol of standardized video transmission is the MDA (monochrome display adapter) protocol that provides monochrome video and sync signals to a monochrome monitor. In this scheme, a 9 terminal, subminature "D" connector is mounted on the computer for coupling the MDA signals to a monitor. Of these terminals, terminals 1 and 2 are connected to logic ground, and terminals 3, 4, and 5 are not used. Terminal 6 provides the intensity signal, terminal 7 provides the video signal, and terminals 8 and 9 provide the horizontal and vertical sync signals (HS, VS), respectively. The video and sync signals are TTL level signals that swing from a HIGH of at least 2.4 volts down to a LOW of at least 0.6 volts.
Another early scheme is the CGA (color graphics adapter) protocol. Here, the computer is again provided with a 9 terminal, subminature "D" type connector, with terminals 1 and 2 being coupled to logic ground. Terminals 3, 4, and 5 carry the red, green, and blue video signals, respectively, with terminal 6 carrying the intensity signal. Terminal 7 is reserved, or not used, and terminals 8 and 9 carry the horizontal and vertical sync signals, respectively. These CGA video signals are also digital TTL level signals.
An enhanced version of the CGA scheme emerged, known as EGA (enhanced graphics adapter), which, while still using TTL video signals, provided better quality color video. This scheme called for the computer to have a subminature "D" connector having 9 terminals, with terminal 1 coupled to logic ground. Terminals 3, 4, and 5 carry the primary video signals red, green, and blue, respectively, and terminals 2, 6, and 7 carry the secondary video signals red, green, and blue, respectively. Terminals 8 and 9 carry the horizontal and vertical sync signals, respectively. As with the above protocols, these EGA signals are digital TTL signals.
A later development in computer video interface signals is the VGA (video graphics adapter) protocol, which uses a 15 terminal, high density (HDD) "D" connector mounted to the computer for providing the VGA signals. Here, terminals 1, 2, and 3 carry the red, green, and blue video signals, and terminals 6, 7, and 8 carry return potentials of the red, green, and blue signals, respectively. Terminals 13 and 14 carry the horizontal and vertical sync signals, respectively, with terminal 10 being a sync return for the vertical and horizontal sync signals. The terminal 8 position is a keyed position having no terminal therein, and terminals 5 and 15 are reserved, or not connected. Terminals 11, 12, and 4 carry identification potentials from the monitor that indicate to the computer the type of VGA monitor; monochrome, color or high resolution, that is coupled to the computer. In contrast to the TTL protocols described above, the VGA video signals red, green, and blue are analog signals having a swing of from about 0-700 millivolts, with the horizontal and vertical sync signals and identification signals still being TTL levels. In all of the described schemes, the outer, metallic shield encasing the connector, and the braided shield encasing the transmission cable are coupled to chassis ground.
There are three methods used to input the various types of video formats used by PC type computers. The most common is to use a cable and circuitry tailored specifically to each type or class of video signals. This approach is used by CYBEX Corporation, of Huntsville, Ala., in their original manual COMMANDER.TM. keyboard and video switch. This method is also used by ROSE ELECTRONICS.TM. of Houston, Tex., in their manual keyboard and video switch, and by RARITAN COMPUTERS, INC .TM., of Belle Meade, N.J.
The major drawback of these systems of the prior art is that as computer systems are upgraded to include computers having a different class of video signals, the cables and electronics must be replaced due to the fact that they are only capable of handling one type of video signal.
Another method of coupling various TTL classes of video signals to analog computers is employed by RARITAN COMPUTERS Inc..TM.. Here, an adapter is connected between the TTL video outputs and the analog input, which adapter converts the TTL signals to analog signals prior to inputting the signals to an analog switching network.
The disadvantage here is that a standard MDA, CGA, EGA or VGA monitor cannot be used because the horizontal and vertical frequencies and signal timing between TTL and VGA classes of video signals, and even between MDA and CGA/EGA types of video signals are not compatible. In this instance, a more sophisticated, expensive, multisynchronous monitor must be used as a display device. However even with this more sophisticated monitor, depending upon the quality of the monitor, manual adjustment of the position of the image and borders may be required.
The third method is to use cables which couple to the TTL or VGA output of the computer at one end and which are connected at the other end to the electronics via a single connector, the circuit card being configurable with jumpers for each respective video type, such as in the AUTOBOOT COMMANDER .TM., introduced by CYBEX CORPORATION, in 1990. A similar product was introduced by ROSE ELECTRONICS in 1992 which did not require configuration by jumpers, but which used separate terminal connections for TTL and analog video signals, with common terminals for the horizontal and vertical sync signals of the various classes of video signals.
However, with the implementation used by ROSE ELECTRONICS, there are insufficient terminals provided in the keyboard and video switch input terminals to carry all the analog signal lines, such as the ID bits and two of the three secondary color lines of the EGA type signals to the electronics. This may result in an incorrect display when using computers having an EGA class output, and may cause initialization failures of some types of computers providing VGA outputs.
The prior art AUTOBOOT COMMANDER is the only instance known by Applicants where the video and keyboard connectors are coupled to a single connector that meets the requirements of the MDA, CGA, EGA, and VGA classes of computer signals. Here, the switching unit receives video signals, keyboard signals, and if used, mouse signals from the respective ports of up to eight computers and couples the ports of one of these computers to a single monitor, keyboard, and mouse. Each of the computers is coupled to a separate circuit card, called a "channel card", within the COMMANDER unit, with each channel card controlled by a microprocessor. As such, up to 12 COMMANDER units may be coupled together to form a system containing up to 96 microcomputers, the microcomputers typically being linked together by a networking scheme. The channel cards may each be manually configured by manipulation and setting of multiposition connectors known as "jumpers", or alternately, setting DIP switches, to configure each channel card to accept signals of one of the aforementioned video protocols.
However, manually configuring these channel cards is not an easy task, as first the computer system must be shut down, disrupting computer service to the users for the period of time necessary to reconfigure the channel card/cards. The COMMANDER unit must then be disconnected from the computers and peripheral devices, disassembled to gain access to the channel cards, and the instruction manual studied in order to ascertain which jumpers to place in what positions. After the jumpers on the channel card/cards are reconfigured, the unit must then be reassembled, and then reconnected to the computers and peripheral devices. Lastly, the system must be energized, or "booted", restoring service to the users. As there are 18 video jumpers on each card, this procedure may take up to an hour or so, depending on the skill level of the individual performing this operation.
More problems arise where different protocols of video signals are present, as where a number of CGA/EGA computers were initially installed in a computer system, and then the system expanded to include VGA computers. With the addition of each type new types of computers, it may be necessary to reconfigure channel card as described, with the necessary time spent to accomplish this task resulting in the attendant disruption of computer services.
Accordingly, it is an object of this invention to eliminate the need to manually configure electronic components for specific tasks or operations. This is achieved by the use of video identification signals and particular cabling configurations that automatically configure switching circuitry that provide the signals as outputs, greatly reducing the time spent installing a discrete computer or computers in a computer system.